The long-range objectives of this research project are to establish the physiological significance of reactions involved in the modification of the NH2-terminal region of newly initiated proteins and to determine whether or not alterations in these reactions are important in disease processes such as cancer and genetic diseases. To accomplish these goals we plan to establish the types of reactions leading to the modification of newly initiated proteins, the relative frequency of these reactions, and whether or not significant differences in these NH2-terminal modifications occur in normal and neoplastic cells. We plan to pursue these goals by: 1) studying the structure of the NH2-terminal region of eukaryotic proteins synthesized in vivo and in vitro, 2) comparing the types and extent of NH2-terminal modification of proteins from normal and neoplastic cells, and 3) identifying, isolating, and characterizing the enzymes involved in these reactions. Our present data indicate that the NH2-terminal region of eukaryotic proteins is modified by the removal of the NH2-terminal methionine from most newly initiated proteins and by the alpha-N-acetylation of as many as 80 percent of these proteins. The reasons for the modification reactions are unknown.